Just gonna dump the top of my Xfire list here...
World of Warcraft - 1 724 hours
Team Fortress 2 - 604 hours
Heroes of Newerth - 382 hours
Warcraft III - The Frozen Throne - 238 hours
Neverwinter Nights 2 - 202 hours
Dragon Age: Origins - 158 hours
Diablo II - Lord of Destruction - 158 hours
Neverwinter Nights - 157 hours
Mass Effect - 146 hours
Mass Effect 2 - 134 hours
StarCraft II - 111 hours
Dawn of War Dark Crusade - 101 hours
But this isn't entirely accurate, since I only got Xfire in November 2006, and it only shows PC games. I'd say about 100-120 more hours on TFT (and even more if you were to count ROC), about a thousand more hours on WoW, and approximately a whole lot more on Diablo II. Also, according to Xfire, I've played about 30 hours of Counter-Strike. Yeah, right. Make that about 200-300 hours, if not more.
Console games I've spent 100 hours on are few and far between, though. I'd say probably Ocarina of Time, Mario 64 and Goldeneye, since they're the only 3 games I had for my Nintendo 64 which I played religiously from grades two to... five? six?
Also Pokemon. I don't want to know how many hours if you put all of it together.
Now stop looking at me like that. I have just enough spare time, thank you very much.